Tail number
The tail number is an aircraft's registration painted on the fuselage or tail, for example N123AB or G-JETX, and it is the single most useful identifier a charter customer can ask for. With it you can look up the owner, the age and model of the aircraft, whether it is on an operator's Part 135 or AOC list, its recent flights on tracking sites and any accident or incident history. A quote that will not commit to a tail number until the day of departure is not necessarily suspicious, since floating fleets swap aircraft, but the operator should be able to name the specific aircraft, or at least the type and operator, before you pay.